There are many instances in which it is desirable to sense the presence and/or quantity or concentration of an analyte in a gas. “Analyte” as the term is used herein is used broadly to mean the chemical component or constituent that is sought to be sensed using devices and methods according to various aspects of the invention. An analyte may be or comprise an element, compound or other molecule, an ion or molecular fragment, or other substance that may be contained within a fluid. In some instances, embodiments and methods, there may be more than one analyte present, and an objective is to sense multiple analytes. “Gas” as the term is used herein also is used broadly and according to its common meaning to include not only pure gas phases but also vapors, non-liquid fluid phases, gaseous colloidal suspensions, solid phase particulate matter or liquid phase droplets entrained or suspended in gases or vapors, and the like. “Sense” and “sensing” as the terms are used herein are used broadly to mean detecting the presence of one or more analytes, or to measure the amount or concentration of the one or more analytes.
In many instances, there is a need or it is desirable to make the analysis for an analyte in the field, or otherwise to make such assessment without a requirement for expensive and cumbersome support equipment such as would be available in a hospital, laboratory or test facility. It is often desirable to do so in some cases with a largely self-contained device, preferably portable, and often preferably easy to use. It also is necessary or desirable in some instances to have the capability to sense the analyte in the fluid stream in real time or near real time. In addition, and as a general matter, it is highly desirable to accomplish such sensing accurately and reliably.
The background matrix of breath presents numerous challenges to sensing systems, which necessitate complex processing steps and which further preclude system integration into a form factor suitable for portable usage by layman end-users. For example, breath contains high levels of humidity and moisture, which may interfere with the sensor or cause condensation within the portable device, amongst other concerns. Also, the flow rate or pressure of breath as it is collected from a user typically varies quite considerably. Flow rate variations are known to impact, often significantly, the response of chemical sensors. Breath, especially when directly collected from a user, is typically at or near core body temperature, which may be considerably different than the ambient temperature. Additionally, body temperature may vary from user to user or from day to day, even for a single user. Devising a breath analyzer thus is a non-trivial task, made all the more difficult to extent one tries to design and portable and field-amenable device.
Notably, the measurement of endogenous analytes in breath presents different challenges and requires different techniques and devices than the measurement of exogenous analytes. Endogenous analytes are those that are produced by the body, excluding the lumen of the gastrointestinal tract, whereas exogenous analytes are those that are present in breath as a result of the outside influence or as a result of user consumption. However, many analytes are produced endogenously and can also be exogenously introduced. For example, ammonia is produced endogenously through the metabolism of amino acids, but can also be introduced exogenously from the environment such as ammonia-containing household cleaning supplies. The term “endogenous” is used according to its common meaning within the field. Endogenous analytes are produced by natural or unnatural means within the human body, its tissues or organs, typically excluding the lumen of the gastrointestinal tract.
There are a number of significant challenges to measuring endogenous analytes in breath. Endogenous analytes typically have significantly lower concentrations in the breath, often on the order of parts per million (ppm), parts per billion (ppb), or less. Additionally, measurement of endogenous analytes requires discrimination of the analyte in a complex matrix of background gases. Instead of typical atmospheric gas composition (e.g., primarily nitrogen), exhaled breath has high humidity content and larger carbon dioxide concentration. This leads to unique challenges in chemical sensitivity, selectivity and stability. For example, chemistries conducive for breath ammonia measurement are preferably sensitive to 50 ppb in the presence of 3 to 6% water vapor with 3 to 5% carbon dioxide.
Because of the historical difficulty in even detecting endogenous breath analytes, other challenges have not been extensively investigated. Examples of such challenges include: (a) correlating the analytes to health or disease states, (b) measuring these analytes given characteristics of human exhalation, e.g., flow rate and expiratory pressure, (c) measuring these analytes sensitively and selectively, and (d) doing all these in a portable, cost effective package that can be implemented in medical or home settings.
Colorimetric devices are one method for measuring a reaction involving a breath analyte. Colorimetric approaches to endogenous breath analysis have historically been plagued with lengthy response times, and expensive components. Often such analysis has to be performed in a laboratory. Thus there remains a need for a breath analyzer that can measure endogenous breath components present in relatively low concentrations, such as acetone, accurately and quickly, without a long wait period for results, in addition to being inexpensive and useable by the layperson. It is also preferable if the breath analyzer is capable of measuring multiple analytes.